Impact of agricultural practices on surface water quality: a case study in the problem area of the Horstgaterbeek in North Limburg, Flanders

In the context of the EU Nitrate directive, a project on surface water quality was started in 2012 in the area of the Horstgaterbeek, a stream in North Limburg, Flanders. Due to its characteristics, this is an ideal area to gain more insight in nitrate flows linked with agricultural activities. The area has an important agricultural activity, with many livestock farms and grassland, but also arable crops. It is a flat area with sandy to sandy-loam soils and shallow groundwater tables. In the area 2 monitoring sites for surface water quality, belonging to the MAP monitoring network of the Flemish Environmental Agency, are situated in small ditches. In one of them, the nitrate content regularly exceeds the standard (50 mg NO3- l-1). In the other site, the nitrate contents remained below the nitrate standard during the last 10 years. The aim of the project is to get a better insight in the nitrate flows in the area and to improve surface water quality, through an intensive follow-up of the agricultural activities, in collaboration with the local farmers. First, the catchments of both MAP monitoring sites were determined, covering a total area of about 150 ha. The drainage in the area through streams and ditches, the agricultural activities, as well as other possible nitrate sources (e.g. households not connected to sewerage) were mapped. Relevant information on the farms, such as farming type, manure production and storage, disposal of manure effluents, cleaning water and domestic sewage, etc., and on the agricultural parcels, such as soil type, organic matter content, chemical fertility, presence of interfering soil layers, cropping and fertilisation history, etc. was collected. In 2012, the project area consisted mainly of maize and grassland parcels. Also sugar beet, peas, beans, fodder beet, potatoes and spring barley was grown. In more than half of the parcels, the C-content as well as the pH were below optimum. The P-content was mostly high. In the agricultural parcels, N-content was determined in spring and fall, manure samples were taken and field activities were registered. All farmers received parcel-specific fertilisation recommendations based on soil analyses. By comparing measured N-contents in fall 2012 and in spring 2013, N-leaching during winter was estimated. At the end of the growing season N- and P-balances were calculated. Beside the 2 existing monitoring sites in the area, the surface water quality was monitored upstream in 3 supplementary sites, also situated in small ditches. Furthermore 8 monitoring sites for groundwater (monitoring wells) were installed. The location of these wells was chosen in such a way that their infiltration areas correspond with one or two agricultural parcels within the project area. Nitrate and phosphate was determined monthly in all monitoring sites. Finally, the link between the measured nitrate and phosphate contents in surface and groundwater and the agricultural practices will be investigated.